1. Field of the Invention
This invention relates to a rear wheel turning system for turning the rear wheels of a vehicle in response to the operation of a steering wheel.
2. Description of the Prior Art
There has been known a four-wheel steering system for a vehicle in which not only the front wheels but also the rear wheels are turned in response to the operation of the steering wheel.
A four wheel steering system generally comprises a front wheel turning mechanism for turning the front wheels and a rear wheel turning mechanism for turning the rear wheels which are mechanically or electrically connected to each other. In a four wheel steering system of the latter type, the rear wheel turning mechanism is driven by an electric motor which is operatively connected to the steering wheel as disclosed in U.S. Pat. No. 4,645,025.
As disclosed in the United States patent, it has been proposed to provide the rear wheel turning mechanism with a rear-wheel-neutralizing means which normally urges the rear wheel turning mechanism into the neutral position in which it holds the rear wheels in a straight-ahead position. The rear-wheel-neutralizing means functions as a fail-safe mechanism which returns the rear wheels to the straight-ahead position and holds them there in case the control system fails.
For such motor-driven rear wheel turning mechanisms, a motor brake is generally provided for the electric motor in order to enhance the holding torque, and a reduction train is generally interposed between the electric motor and the rear wheel turning mechanism in order to allow a smaller electric motor to be used. Accordingly, in order to return the rear wheels to the straight-ahead position, the rear-wheel-neutralizing means must overcome the holding torque of the electric motor which is transmitted thereto by way of the reduction train, and therefore, it is very difficult for the rear-wheel-neutralizing means to actually return the rear wheels to the straight-ahead position and hold them there. This problem may be overcome by the use of a clutch which is interposed between the electric motor and the rear wheel turning means and is adapted in such a way that it disconnects the latter from the former and permits the latter to return to the neutral position under the force of the rear-wheel-neutralizing means. However, if the clutch fails or jams, or the circuit for energizing the clutch is broken, the clutch cannot be disengaged, and accordingly, the rear wheels cannot be correctly returned to the straight-ahead position.